Digestive Function

The common purpose of digestion is to convert food into smaller nutrient molecules that can be absorbed across the wall of the digestive tract. The efficiency of digesting food (i.e., digestibility) depends on both the quality of the food and the digestive function of the consumer. Digestibility determines how much of a food will be needed to meet the animal’s energy and nutrient requirements for maintenance and production. Functional diet breadth is determined by the lowest digestibility at which an animal can meet its requirements. Increases in gut capacity allow animals to process more food of lower digestibility, whereas increases in energy requirements (e.g., for growth or reproduction) may force an animal to select a narrow range of highly digestible foods.

The headgut (the mouth and pharynx) prepares food for digestion by mechanical disruption (chewing) and solvation. The foregut (esophagus and stomach) is also involved in mechanical disruption of food, and the initial stages of digestion. Digestion of simple foods by endogenous enzymes occurs mainly in the tubular midgut (small intestine). Digestion of plant material occurs by microbial enzymes in fermentation chambers that are expansions of either the foregut (forestomach) or hindgut (cecum and colon). Digesta flow affects the rates of digestion in the tubular midgut as well as in the fermentation chambers. A single mixing vat in the rumen provides more thorough digestion of fibrous particles, but a series of chambers in the colon allows faster flows of particles when food quality declines.